Occupy one unit, rent the rest
House hacking means you buy a small multifamily — usually a duplex, triplex, or fourplex — live in one unit, and lease the others. Owner-occupy financing is the point: the down payment is often much smaller than an investor loan on the same building.
BrickAtlas models that as a 2–4 unit owner-occupy. Rent in the underwrite is market rent for one unit you do not live in, multiplied by the other units. Your own unit does not pay you rent.
The number that decides it
Effective housing cost is the monthly mortgage plus operating costs, minus rent from the other units. Below zero, those units cover the building and leave cash flow. Above zero, that remainder is what it costs you to live there.
Operating costs on this screen are taxes, insurance, HOA, and land or lot rent for the whole property, plus maintenance and management as a percent of the other units’ rent only. Vacancy is not subtracted. If the market turns slowly, lower the rent before you trust the offset.
- Mortgage uses the owner-occupy down payment. The house-hack model defaults to 5% of price. Change it to match the loan you can actually get.
- Cash required is that down payment, plus half the rehab budget, plus purchase closing. Purchase closing defaults to 3% of price.
- A full rental underwrite counts the entire rehab budget and haircuts rent for vacancy. Run that too if you might move out.
- Cash-on-cash here is the surplus after housing, divided by cash required. It is not the yield you would earn renting every unit.
Shapes that use the same idea
The calculator and the Investment Memo use the 2–4 unit model. Two cousins follow the same offset, with different operations.
- Duplex, triplex, or fourplex — live in one, lease the others. This is the model to screen.
- Single-family with roommates — rent from rooms instead of units. More turnover, and the loan may still be a one-unit primary residence.
- An accessory dwelling — you add a unit, then rent it. That is a different strategy in Analyst, with build cost and value add, not just an occupancy split.
Where house hacks break
The pretty version assumes the other units stay filled and you stay put. Both are choices, not facts.
Owner-occupy programs usually require you to live in the property. If the plan is to move out in a year, underwrite the building as a normal rental before you buy: every unit rented, vacancy in, full rehab cash in. A house hack that only works while you occupy is a housing subsidy, not a portfolio.
One vacant unit can put the mortgage back on you. Size the payment to what you can carry with a door empty. Insurance, local rental rules, and the work of living next to your tenant are part of the deal, not footnotes.
Checklist before you write an offer
Use this gate before you treat the offset as spendable:
- Unit count and occupancy match a loan you can close, not a screenshot of someone else’s terms.
- Rent on the other units is market rent, documented, and already reduced if vacancy is likely.
- Taxes and insurance are for the whole building.
- You can carry the mortgage if one rented unit goes empty.
- You have a rental underwrite for the day you move out.
- The metro still fits the persona after the housing discount is ignored.